Damage Build-up in Zirconium Alloys Mechanical Processing and Impacts on Quality of the Cold Pilgering Product

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2 Damage Build-up in Zirconium Alloys Mechanical Processing and Impacts on Quality of the Cold Pilgering Product ASTM 16th International Symposium on Zirconium in the Nuclear Industry Chengdu, China, A. Gaillac 1, C. Lemaignan 2 and P. Barberis 1 1 AREVA / CEZUS Research Center 2 CEA Grenoble / Grenoble INP, Phelma, Simap laboratory ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.2

3 Ingots Cladding tubes forming route Hot Forging Quenching Piercing Hot Extrusion Cold Pilgering Cladding tubes Ingots ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.3

4 Ingots Cladding tubes manufacturing Hot Forging Quenching Piercing Hot Extrusion Cold Pilgering Hot extruded tubes Cladding tubes ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.4

5 Ingots Cladding tubes manufacturing Hot Forging Quenching Piercing Hot Extrusion Cold Pilgering Cladding tubes Cladding tubes ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.5

6 The cold pilgering process Voids in a cold pilgered tube Cracks in a cold pilgered tube What are the mechanisms responsible for the damage of zirconium alloys during the cold pilgering process? How to improve the forming route of cladding tubes to avoid these cold pilgering defects? ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.6

7 Damage build-up in zirconium alloys mechanical processing and impacts on quality of the cold pilgering product Hot extruded tubes Outline Low cycle fatigue damage The cold pilgering process Cladding tubes ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.7

8 Studied materials: Zircaloy 4 (Z4), M5 TM and an experimental quaternary alloy (Q) (% wt) Z4 Fe 0.2 Cr 0.1 Nb - Sn 1.3 O 0.13 Fe, Cr and Nb are in the form of precipitates M5 TM Q Sn and O are in solid solution Studied microstructures: - Hot extruded microstructure: Specimens from hot extruded tubes for Z4, M5 TM and Q - Recristallized microstructure: Specimens from recristallized bars for Z4 and M5 TM See ASTM paper ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.8

9 Z R T 50 µm 50 µm 50 µm Z4 M5 TM Q R Z T Hot extruded microstructures: u Elongated grains in the axial direction (around 50µm long and 10µm wide) u Many small recristallized grains for Z4 ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.9

10 Z R. T Z4 M5 TM Q (0002) Z T Hot extruded textures: R u C axis is perpendicular to the extrusion direction, oriented between 60 and 90 from the radial direction ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.10

11 Precipitates Z4 M5 TM Q Type Intermetallic Zr(Fe,Cr) 2 (low ductility) Metallic βzr and βnb (high ductility) Intermetallic Zr(Fe,Cr) 2 and bigger (Zr,Nb) 4 Fe 2 (low ductility) Metallic βzr and βnb (high ductility) Size Average 170 nm Max 400 nm Average 200 nm Average 200 nm Max 1 µm Precipitates in hot extruded tubes: u Aligned in the extrusion direction u Ductile precipitates in M5 TM u Non-ductile precipitates in Z4 and Q ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.11

12 Damage build-up in zirconium alloys mechanical processing and impacts on quality of the cold pilgering product Hot extruded tubes Outline Low cycle fatigue damage The cold pilgering process Cladding tubes ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.12

13 ε r ε t ε r ε t As a function of its position in the grooves of the rolls, material is: u strained up to 5% at each stroke u strained alternately in compressive and tensile modes Low cycle fatigue deformation route ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.13

14 Low cycle fatigue tests results 10 u Specimens taken in the axial directions of the tubes De 2 p = A. N -a r Δεp/2 (%) 1 Z4 M5 0, Number of cycles to rupture (N r ) Z4 and M5 TM specimens from extruded tubes exhibit the same fatigue life ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.14

15 Low cycle fatigue tests results 10 u Specimens taken in the axial directions of the tubes De 2 p = A. N -a r Δεp/2 (%) 1 Z4 M5 Q 0, Number of cycles to rupture (N r ) Z4 and M5 TM specimens from extruded tubes exhibit the same fatigue life Q alloy specimens have a 30% lower fatigue life than Z4 and M5 TM ones ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.15

16 Z4 Q Final crack propagation Fatigue crack propagation The lower toughness of Q alloy must explain its lower fatigue life Toughness of studied materials have to be quantified ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.16

17 Toughness tests results JIC (kn/m = kj/m 2 ) Z4 M5 Q Toughness of Z4 and M5 TM specimens is equal Toughness of Q alloy is lower ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.17

18 Fracture surface of toughness test samples Dimples on the fracture surface at the crack tip Nucleation of voids Growth of voids Coalescence of voids Stress triaxiality effect? ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.18

19 Damage build-up in zirconium alloys mechanical processing and impacts on quality of the cold pilgering product Hot extruded tubes Outline Low cycle fatigue damage The cold pilgering process Cladding tubes ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.19

20 Macroscopic behavior u Specimens taken in the axial directions of the tubes Strain to rupture 1 0,8 0,6 0,4 0,2 0 Z4 M5 Q Tensile tests: Ductility of Q alloy is less than the Z4 and M5 TM ones ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.20

21 Macroscopic behavior 1 0,8 Z4 Strain to rupture 0,6 0,4 0,2 M5 R 2 R ,0 0,5 1,0 1,5 2,0 Stress triaxiality 1/ Stress triaxiality Stress triaxiality lowers the ductility Same results for Z4 and M5 TM specimens ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.21

22 Macroscopic behavior 1 Strain to rupture 0,8 0,6 0,4 0,2 Z4 M5 Q R 2 R ,0 0,5 1,0 1,5 2,0 Stress triaxiality 1/ Stress triaxiality Stress triaxiality lowers the ductility Same results for Z4 and M5 TM specimens Ductility of Q alloy is less than Z4 and M5 TM ones ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.22

23 Microscopic behavior Z4 ext Z R 50 µm SEM observations of the voids under the fracture surface ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.23

24 Microscopic behavior Z4 M5 TM Q Z R 50 µm Differences of size, number and distribution of voids ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.24

25 Microscopic behavior Z4 2 Z R 1 0 Voids volume fraction (%) Mapping of the voids volume fraction under the fracture surface ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.25

26 Microscopic behavior 1 Voids volume fraction (%)XXX 0,8 0,6 0,4 0,2 Z4 Z4_N2 M5 M5_N2 Q Q_N ,2 0,4 0,6 0,8 1 Plastic strain Useful data for ductile damage models used with the numeric simulations of the cold forming processes ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.26

27 Microscopic behavior 1 Voids volume fraction (%)XXX 0,8 0,6 0,4 0,2 Z4 Z4_N2 M5 M5_N2 Q Q_N ,2 0,4 0,6 0,8 1 Plastic strain Nucleation, growth and coalescence of voids have to be plotted on different graphs ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.27

28 mechanism: Nucleation of voids Mean number of voids per image (247 x 168 µm) Z4 M ,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 Plastic strain Number of voids in Z4 is higher than in M5 TM ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.28

29 mechanism: Nucleation of voids Mean number of voids per image (247 x 168 µm) Z4 M5 Q 0 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 Plastic strain Number of voids in Q alloy is much higher than in Z4 et M5 TM alloys Voids nucleate sooner in the Q alloy ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.29

30 mechanism: Nucleation of voids Mean number of voids per image (247 x 168 µm) Z4 Z4_N2 M5 M5_N2 Q Q_N ,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 Plastic strain Voids nucleate sooner with a higher stress triaxiality ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.30

31 mechanism: Nucleation of voids Z R Tensile direction EDX analysis of voids in Z4 For Zircaloy-4 and Q alloy: nucleation of voids on the intermetallic precipitates ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.31

32 mechanism: Nucleation of voids Z R For M5 TM : nucleation of voids on the grain boundaries of highly disoriented grains 5 µm EBSD Inverse pole figure in the radial direction Tensile direction ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.32

33 mechanism: Nucleation of voids Effect of the microstructure on the nucleation of voids : u Different voids nucleation kinetics between Z4 and M5 TM Different voids nucleation mechanism (SPP for Z4, grain to grain disorientations for M5) u Number of voids in Q alloy is much more higher than in Z4 et M5 TM alloys Higher number of precipitates in the Q alloy (high iron content) u Voids nucleate sooner in the Q alloy Bigger size of the (Zr,Nb)4Fe2 precipitates ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.33

34 mechanism: Growth of voids 2 Mean diameter of voids (µm) 1,8 1,6 1,4 1,2 1 0,8 0,6 Z4 M5 0,4 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 Plastic strain Voids are bigger in M5 TM than in Z4 ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.34

35 mechanism: Growth of voids 2 Mean diameter of voids (µm) 1,8 1,6 1,4 1,2 1 0,8 0,6 Z4 M5 Q 0,4 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 Plastic strain Voids are smaller in the Q alloy ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.35

36 mechanism: Growth of voids 2 Mean diameter of voids (µm) 1,8 1,6 1,4 1,2 1 0,8 0,6 Z4 Z4_N2 M5 M5_N2 Q Q_N2 0,4 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 Plastic strain Growth rate of voids is higher with a higher stress triaxiality ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.36

37 mechanism: Growth of voids Z R Tensile direction EBSD Inverse pole figure in the radial direction Z4 rx Growth of voids is stopped or slowed by the grain boundaries So, growth rate is higher for bigger grains ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.37

38 mechanism: Growth of voids Effect of the microstructure on the growth of voids : u Voids are bigger in M5 TM than in Z4 Bigger grains in M5 TM than in Z4 u Voids are smaller in the Q alloy Higher number of voids earlier coalescence for the Q alloy ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.38

39 mechanism: Coalescence of voids Growth of voids Mean distance between voids, perpendicularly to the tensile direction (µm) ,5 1 1,5 2 2,5 3 Nucleation of voids Z4 ext M5 ext Mean diameter of voids (µm) ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.39

40 mechanism: Coalescence of voids Growth of voids Mean distance between voids, perpendicularly to the tensile direction (µm) ,5 1 1,5 2 2,5 3 Nucleation of voids Z4 rx Z4 ext M5 ext Mean diameter of voids (µm) ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.40

41 mechanism: Coalescence of voids Growth of voids Mean distance between voids, perpendicularly to the tensile direction (µm) ,5 1 1,5 2 2,5 3 Nucleation of voids Z4 rx Z4 ext M5 rx M5 ext Mean diameter of voids (µm) ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.41

42 mechanism: Coalescence of voids Growth of voids Mean distance between voids, perpendicularly to the tensile direction (µm) Coalescence and rupture 0 0,5 1 1,5 2 2,5 3 Nucleation of voids Z4 rx Z4 ext M5 rx M5 ext Q ext Mean diameter of voids (µm) Coalescence criterion: - is not a function of the alloy or the microstructure ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.42

43 mechanism: Coalescence of voids Growth of voids Mean distance between voids, perpendicularly to the tensile direction (µm) Coalescence and rupture 0 0,5 1 1,5 2 2,5 3 Nucleation of voids Z4 rx Z4 ext Z4 ext_n2 M5 rx M5 ext M5 ext_n2 Q ext Q ext_n2 Mean diameter of voids (µm) Coalescence criterion: - is not a function of the alloy or the microstructure - is a function of the stress triaxiality ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.43

44 mechanism: Metallurgical parameters Precipitates size (Z4, Q) Grain to grain disorientations (M5 TM ) Precipitates number (Z4, Q) Grain size Texture (see ASTM paper) Voids nucleation sites distribution (see ASTM paper) Voids Nucleation strain threshold Number Growth rate Strain coalescence threshold Mechanical parameters Stress triaxiality ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.44

45 General conclusion Better understanding of the cold damage mechanisms for various alloys and metallurgical states, effect of the: u Precipitates u Grain to grain disorientations u Grain size u Texture (see ASTM paper) u Industrial applications: u Quantified data on ductile damage of Zr alloys, used with the numerical simulations of the cold pilgering process u Optimization of the cold forming routes of the tubes (see ASTM Poster) ASTM 16th International Symposium on Zirconium in the Nuclear Industry - CEZUS Proprietary - All rights reserved - CEZUS - p.45

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